Devices which provide a reciprocating motion through an actuator member as that member is moved through an actuation stroke are known in the prior art. Such actuators may be fluid powered--hydraulically or pneumatically--or electrically powered. An advantage to a hydraulic powered actuator is that it provides high linear forces in an extremely small packaging, typically cylinder/piston arrangement. Hydraulic systems, no matter how well maintained, leak fluid contributing to an ever growing environment problem. While achievable, computer control of a hydraulic cylinder actuator involves substantial complexity. Pneumatic driven actuators while clean are nearly impossible to place under electronic control due to air's compressibility.
An electric motor, which may be employed to develop the fluid power for the fluid systems described above, is a highly efficient and reliable system for delivering usable power. In the context of a fluid system, however, much of the efficiency is lost in the generation of the fluid power. Additionally, computer control of an electric motor is relatively simple. For this and other reasons mentioned above, the trend has been to directly link the rotating output shaft of an electric motor to a mechanical device which converts the rotational motion into reciprocal or linear motion. The most common method is to use a lead screw coupled directly to the motor output shaft with a follower unit traveling along the threads of the lead screw.
The motor/lead screw combination addressees some of the difficulties of a fluid powered system. However, at the point of application of the reciprocating motion, this combination requires considerably more space. Typical prior art arrangements also result in exposure of the threaded components and/or nonconcentric connection of the lead screw and follower assembly to the load.
An example of one prior art system is disclosed in U.S. Pat. No. 2,683,379 (Strandgren) in which a reciprocating member is enclosed by a threaded drive element. Typically, a motor assembly is coupled to an end of such a drive element contributing to the overall length of the actuator unit.
U.S. Pat. No. 3,660,704 (Paine et al.) discloses a motor concentrically mounted about a threaded drive element. This reduces the length of the actuator assembly unit in short stroke applications, and encloses the threads. However, it substantially lengthens the actuator in long stroke applications since a smooth section equal in length to the threaded section must be included in order for the actuator shaft to slide through the bushing.
In addition to providing a linear actuator which addresses the foregoing shortcomings, a need exists for a linear actuator apparatus and system which offers precise positioning control. Further, there also exists a need for a linear actuator including a volumetric pump method and apparatus.